anees1221,
If you can see the drive in the Disks utility, but not in the File Manager, then it may not be mounted. Using the Disks utility, try highlighting the drive on the left hitting the Play button (sideways triangle button under the Volumes section) on the right to mount it. If it's already mounted the Play button will look like a stop button (small black square). Otherwise, you might have some formatting/corruption issues that would be better addressed by viewing this YouTube video:
Or if you're using Windows, view this: